U.S. patent number 5,431,759 [Application Number 08/199,904] was granted by the patent office on 1995-07-11 for cable jacketing method.
This patent grant is currently assigned to Baker Hughes Inc.. Invention is credited to David H. Neuroth.
United States Patent |
5,431,759 |
Neuroth |
July 11, 1995 |
Cable jacketing method
Abstract
An electrical cable and a method for manufacturing the
electrical cable are provided in which a plurality of insulated
conductors have a protective jacket extruded thereabout, the
protective jacket having an exterior ribbed surface which includes
a plurality of longitudinally extending ribs between which extend a
plurality of thermal expansion voids. The protective jacket is
formed from a thermally set elastomeric material which is partially
cured, and then a protective exterior armor is helically wrapped
around the exteriorly ribbed surface of the elastomeric, protective
jacket. Then, the electric cable is heated to an elevated
temperature for a period of time which is sufficient for fully
curing the elastomeric protective jacket formed therein.
Inventors: |
Neuroth; David H. (Tulsa,
OK) |
Assignee: |
Baker Hughes Inc. (Houston,
TX)
|
Family
ID: |
22739495 |
Appl.
No.: |
08/199,904 |
Filed: |
February 22, 1994 |
Current U.S.
Class: |
156/53; 156/169;
156/307.1; 156/307.7; 156/54; 174/102R; 174/109; 264/171.15;
264/172.15; 264/176.1; 264/177.17 |
Current CPC
Class: |
H01B
7/046 (20130101); H01B 13/14 (20130101); H01B
13/145 (20130101) |
Current International
Class: |
H01B
13/14 (20060101); H01B 7/04 (20060101); H01B
13/06 (20060101); H01B 013/08 (); H01B 013/14 ();
H01B 013/26 () |
Field of
Search: |
;174/12R,109,13,107
;156/48,51,52,53,54,169,307.1,307.7,309.9
;264/174,176.1,177.17 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ball; Michael W.
Assistant Examiner: Lorin; Francis J.
Attorney, Agent or Firm: Bradley; James E. Handley; Mark
W.
Claims
What is claimed is:
1. A method for manufacturing an electric cable for use in a
wellbore to conduct electrical power to a downhole submersible
pump, said method comprising, in the following order, the steps
of:
providing a plurality of insulated conductors which provide, at
least in part, a longitudinally extending central core;
extruding a protective jacket about said plurality of insulated
conductors to encapsulate said plurality of insulated conductors
therein;
curing in a continuous vulcanizing process only an exterior portion
of said protective jacket by heating only said exterior portion to
at least a cure temperature for a first time period of a limited
duration to prevent an interior portion of said protective jacket
from heating to said cure temperature for a sufficient interval of
time to fully cure;
wrapping a metal armor exteriorly around said protective jacket to
form said electric cable;
spooling said electric cable onto a reel; and
placing said reel containing said electric cable into an oven for a
time and temperature sufficient to fully cure said interior portion
of said protective jacket.
2. The method of manufacturing an electric cable of claim 1,
wherein said continuous vulcanizing process comprises:
feeding said protective jacket and insulated conductors into a
vulcanizing tube which contains a pressurized steam at a
temperature of substantially not less than 177.degree. C. (350
degrees Fahrenheit; and
passing said protective jacket and insulated conductors through
said vulcanizing tube at a rate of speed which is not substantially
less than thirty meters per minute (one hundred feet per
minutes).
3. The method of manufacturing an electric cable of claim 1,
wherein said temperature of said oven is not substantially less
then 116.degree. C. (240 degrees Fahrenheit) and said time within
said oven is not substantially less than twenty-four hours.
4. The method of manufacturing an electric cable of claim 1,
wherein said step of extruding said protective jacket about said
sleeve includes providing an exteriorly ribbed surface having a
plurality of ribs with a plurality of thermal expansion voids
therebetween.
5. The method of manufacturing an electric cable of claim 1,
wherein said step of extruding said protective jacket about said
sleeve includes providing an exteriorly ribbed surface having a
plurality of longitudinally extending ribs with a plurality of
longitudinally extending thermal expansion voids therebetween.
6. A method for manufacturing an electric cable for use in a
wellbore to conduct electrical power to a downhole submersible
pump, said method comprising, in the following order, the steps
of:
providing a plurality of insulated conductors which provide, at
least in part, a longitudinally extending central core;
extruding a protective jacket about said plurality of insulated
conductors to encapsulate said plurality of insulated conductors
therein;
passing said protective jacket and said insulated conductors
through a continuous vulcanizing tube to heat and fully cure only
an exterior portion of said protective jacket and to not cure an
interior portion of said protective jacket, said exterior portion
which is fully cured having a radial thickness substantially in a
range between 0.625 to 1.270 millimeters (twenty-five thousandths
of an inch and fifty thousandths of an inch);
wrapping a metal armor exteriorly around said protective jacket to
form said electric cable;
spooling said electric cable onto a reel; and
placing said reel containing said electric cable into an oven for a
time and temperature sufficient to fully cure said interior portion
of said protective jacket.
7. The method of manufacturing an electric cable of claim 6,
wherein said continuous vulcanizing process comprises:
feeding said protective jacket and insulated conductors into a
vulcanizing tube which contains a pressurized steam at a
temperature of substantially not less than 176.degree. C. (350
degrees Fahrenheit); and
passing said protective jacket and insulated conductors through
said vulcanizing tube at a rate of speed which is selected to
expose a point on the cable to the steam for approximately two
minutes to two minutes, forty-five seconds.
8. The method of manufacturing an electric cable of claim 6,
wherein said temperature of said oven is not substantially less
then 116.degree. C. (240 degrees Fahrenheit), and said time within
said oven is not substantially less than twenty-four hours.
9. The method of manufacturing an electric cable of claim 6,
wherein said step of extruding said protective jacket about said
sleeve includes providing an exteriorly ribbed surface having a
plurality of ribs with a plurality of thermal expansion voids
therebetween.
10. The method of manufacturing an electric cable of claim 6,
wherein said step of extruding said protective jacket about said
sleeve includes providing an exteriorly ribbed surface having a
plurality of longitudinally extending ribs with a plurality of
longitudinally extending thermal expansion voids therebetween.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to electrical cables for use in
hostile environments, and in particular to an electrical cable for
use in an oil and gas well to conduct electrical power to a
downhole submersible pump.
2. Description of the Prior Art
This invention concerns electrical cables of the type which are
used to power downhole electric motors for submersible pumps within
oil and gas wells. These submersible pumps normally pump a mixture
of oil and brine from wells often several thousand meters (feet)
deep and often under high temperatures and pressures. The
electrical cables normally consist of three stranded or solid
conductors. Each stranded or solid conductor contains an insulating
layer of a material that is resistant to oil and brine. Typically,
in a round configuration, an elastomeric, protective jacket is
extruded to cover all three conductors, and an outer metallic armor
surrounds the jacket.
The elastomeric, protective jacket is typically formed from a
thermally set elastomeric material and is used to provide a seal to
prevent wellbore fluids from reaching a thermoplastic insulation
which forms a sleeve about each of the electrical conductors within
the electric cable. Prior art thermally set elastomeric protective
jackets are cured either before or after adding the exterior armor
by heating the jacket to an elevated temperature for a period of
time sufficient to vulcanize the thermally set elastomeric material
from which the protective jacket is formed. Please note that the
term vulcanize, as used herein, does not necessarily indicate the
use of sulfur to cure the elastomeric material.
The period of time required to fully cure a protective jacket about
insulated conductors is determined by the combination of time: to
first heat the entire elastomeric material to a cure temperature,
and then, the length of time at which the elastomeric material must
remain at that temperature to fully cure. Typically, the
elastomeric material within the interstices between the insulated
conductors is the last portion of the protective sleeve to be
heated to the cure temperature, and consequently the last portion
of elastomeric material to cure.
One method of curing the thermally set elastomeric material which
provides the protective jacket is in a continuous cure process,
such as a continuous vulcanization process. One typical prior art
continuous vulcanization process included a vulcanization tube
which was 91 meters (300 feet) long, of which about two-thirds of
the length was filled with pressurized steam at a temperature of
204.degree. C. (400 degrees Fahrenheit) and a pressure of 1.7
megapascals (250 pounds per square inch). The thermally set
elastomeric material was extruded about the insulated conductors to
form a protective jacket, and then passed through the vulcanization
tube at a rate of speed of between 7.6 to 9.1 meters per minute (25
to 30 feet per minute). This rate of speed was selected to retain
the elastomeric material within the steam filled portion of the
tube for a long enough period of time to fully cure a protective
jacket having an outer diameter of roughly 30 millimeters (one and
three sixteenths inches). Of course, the rate of speed at which
different sizes of cable can be cured by passing through the same
length of vulcanization tube changes with the thickness and heat
capacities of the materials to be cured.
A problem with continuous vulcanization processes arises in that
the elastomeric material of a protective jacket must be retained
within the tube for a period of time which is long enough to heat
the entire protective jacket to a cure temperature, and then
retained at this cure temperature for a sufficient length of time
to fully cure the elastomeric material within the interstices of
the insulated conductors. For a specific length of vulcanization
tube, the dwell time at which the protective jacket is retained
therein determines the speed at which the manufacturing process may
be operated. If the dwell time at which the protective jacket is
retained within the vulcanization tube could be decreased, in
general, the manufacturing process could be operated at a faster
rate, and thus improve productivity of the production process.
One way to improve the productivity of the manufacturing process is
to batch cure the protective jacket by spooling the cable onto
reels, and heating an entire length of cable within an oven. One
such example is U.S. Pat. No. 4,675,474, issued on Jun. 23, 1987,
and invented by David H. Neuroth, in which an elastomeric,
protective jacket was cured after armoring and spooling the cable
onto a reel. However, when a protective jacket is batch cured after
spooling onto a reel, the insulated conductors therein may not be
held in proper position, centered within the protective jacket, but
rather may shift to one side. Shifting of insulated conductors
within the protective jacket may reduce the wall thickness of
elastomeric material about the conductors to a thickness which is
insufficient for reliably providing a fluid barrier to prevent
wellbore fluid from attacking the insulating material about the
conductors.
Another problem with prior art electric cables arises since some
electric cables are cured prior to helically wrapping an exterior
armor about the protective jacket. Wrapping the exterior armor
about a fully cured protective jacket results in applying internal
compressive stresses to the jacket material.
Further, in prior art electric cables, the protective jacket is
disposed inside of a metal, exterior armor. The elastomeric
material forming the protective jacket typically has a higher
coefficient of thermal expansion than metal from which the metal,
exterior armor is formed. When prior art electric cables are heated
to high temperatures found downhole within wellbores, the
protective jacket will expand at a greater rate than the exterior
armor. The greater rate of thermal expansion of the protective
jacket within the armor creates compressive forces which act to
apply potentially destructive stresses to the insulation about the
electrical conductors.
Some prior art electric cables include longitudinally extending
ribs about the exterior surface of the protective jacket. These
longitudinally extending ribs provide expansion voids for the
elastomeric material of the protective jacket to expand into when
heated to wellbore temperatures, and thus aid in reducing thermally
induced compressive stresses. When an exterior armor is helically
wrapped about this exteriorly ribbed surface after the protective
jacket is fully cured, the longitudinally extending ribs are
flattened in a helically spiralled pattern. Flattening of ribs
during the manufacturing process changes the location of thermal
expansion voids, resulting in nonuniform compressive stresses when
the electric cable is heated to downhole temperatures within a
wellbore.
SUMMARY OF THE INVENTION
The above as well as additional objects, features, and advantages
of the invention will become apparent in the following detailed
description.
An electrical cable and a method for manufacturing the electrical
cable are provided in which a plurality of insulated conductors
have a protective jacket extruded thereabout. The protective jacket
has an exterior ribbed surface which includes a plurality of
longitudinally extending ribs between which extend a plurality of
thermal expansion voids. The protective jacket is formed from a
thermally set, elastomeric material which is only partially cured
during a continuous vulcanization process. The ribs and a thin
layer underneath the ribs are fully cured. The remaining portion,
which includes the interstices between the insulated conductors, is
not cured. This partial curing is performed by speeding up the
continuous vulcanization process to a rate above that required to
fully cure the protective jacket. The residence time in the hot
portion of the vulcanization tube is thus reduced.
Then a protective exterior armor is helically wrapped around the
exteriorly ribbed surface of the elastomeric, protective jacket.
Compressive forces arising from helically wrapping the exterior
armor about the protective jacket are uniformly distributed and
dispersed through an interior portion of the elastomeric protective
jacket since the interior portion of the elastomeric jacket has not
been fully cured. The ribs do not completely flatten because of the
central uncured portion of the protective jacket. Then, the
electric cable is coiled onto a reel. The reel is subsequently
placed in an oven and the electric cable is heated to an elevated
temperature for a period of time which is sufficient for fully
curing the elastomeric, protective jacket.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features believed characteristic of the invention are set
forth in the appended claims. The invention itself however, as well
as a preferred mode of use, further objects and advantages thereof,
will best be understood by reference to the following detailed
description of an illustrative embodiment when read in conjunction
with the accompanying drawings, wherein:
FIG. 1 is a fragmentary perspective view of an electrical cable for
use in a wellbore to conduct electrical power to a downhole
submersible pump;
FIG. 2 is cross-sectional view of the electrical cable of FIG. 1,
taken along section lines A--A;
FIG. 3 is block diagram partially depicting the preferred method of
manufacturing the electrical cable of the preferred embodiment of
the present invention;
FIG. 4 is fragmentary one-quarter longitudinal section view
depicting a prior art electrical cable which is under compressive
stresses induced by applying an exterior armor; and
FIG. 5 is a fragmentary one-quarter longitudinal section view
depicting the electrical cable of the preferred embodiment of the
present invention after application of an exterior armor.
DETAILED DESCRIPTION OF THE INVENTION
With reference to FIG. 1, a fragmentary perspective view depicts
electrical cable 15 for use in a wellbore to connect electrical
power to a downhole submersible pump. Electrical cable 15 includes
plurality of insulated conductors 17. In the preferred embodiment
of the present invention, there are three insulated conductors 17.
Each insulated conductor 17 includes an electrical conductor 19,
and insulation 21 for preventing electrical current from passing
between electrical conductors 19. Protective jacket 23 is formed
about insulated conductors 17. Protective jacket 23 is provided to
seal insulated conductors 17 to prevent wellbore fluids from
deteriorating either insulation 21 or electrical conductors 19. A
metal exterior armor 25 is helically wrapped about protective
jacket 23 to protect protective jacket 23 against abrasion and to
prevent crushing of insulated conductors 17 and protective jacket
23.
Typically, protective jacket 23 is a singular, continuous member
formed of thermally set elastomeric material. In the preferred
embodiment of the present invention, protective jacket 23 is formed
from materials which are well known in the art, such as, for
example, E.P.D.M. (Ethylene-Propylene-diene Monomers), or nitrile
rubber. In the preferred embodiment of the present invention,
electrical conductors 19 are solid copper conductors, and
insulation 21 is formed from thermoplastic material which are well
known in the art, such as, for example, E.P.D.M.
(Ethylene-Propylene-diene Monomers), or polypropylene. Protective
jacket 23 includes an exteriorly ribbed surface 27. Exteriorly
ribbed surface 27 is formed by plurality of longitudinally
extending ribs 29.
Referring now to FIG. 2, a cross-sectional view depicts electrical
cable 15, and is taken along section line A--A of FIG. 1. A
plurality of thermal expansion voids 31 extend between
longitudinally extending ribs 29. Thermal expansion voids 35
provided space for protective jacket 23 to expand into when
electrical cable 15 is heated to downhole wellbore temperatures. It
should be noted, that exterior armor 25 is typically formed of
metallic material, and protective jacket 23 is typically formed of
elastomeric material which thermally expands at a greater rate than
metallic materials when heated to downhole wellbore temperatures.
Thermal expansion voids 31 provide a place for protective jacket 23
to expand into as it is thermally expanded by a greater amount than
exterior armor 25, and thus serve to reduce thermally induced
compressive forces within protective jacket 23. The expansion voids
31 are uniformly spaced around the exterior of protective jacket
23.
Exterior armor 25 is applied about protective jacket 23 by
helically wrapping a continuous strip 33 of exterior armor 25.
Continuous strip 33 includes a flat underlying end 35 over which
overlaying end 37 is lapped as exterior armor 25 is helically
wrapped about protective jacket 23. Additionally, adhesive 39
secures plurality of insulated conductors 17 together, which are
helically wrapped in a longitudinally extending direction.
With reference to FIG. 3, a block diagram schematically depicts the
preferred method of manufacturing the electric cable of the
preferred embodiment of the present invention. Starting with block
41, insulated conductors 17 are joined by helically twisting the
insulated conductors together. Then, as depicted by block 43,
protective jacket 23 is extruded about the plurality of insulated
conductors 17, by an extruding head which is well known in the
prior art. Insulated conductors 17 and protective jacket 23 then
pass through a continuous curing apparatus to partially cure
protective jacket 23 as depicted by block 45.
The amount of time in the curing apparatus is controlled to fully
cure longitudinally extending ribs 29 and a thin layer of
elastomeric material underneath. In the preferred embodiment of the
present invention, protective jacket 23 has an outside diameter of
approximately 30 millimeters (one and three sixteenths inches),
with longitudinally extending ribs approximately 0.794 millimeters
(one thirty-seconds inches) long. The radial distance from the base
of the ribs 29 to the exterior of the insulation 21 is about 1.270
millimeters (fifty thousandths (0.050) of an inch). The thin layer
of elastomeric material underneath ribs 29 which is fully cured
ranges in radial thickness from approximately 0.635 to 1.270
millimeters (twenty-five thousandths (0.025) to fifty thousandths
(0.050) of an inch). Beneath this thin layer is a transition layer
of partially cured elastomeric material, beneath which is a fully
uncured layer of elastomeric material.
In the preferred embodiment of the present invention, the
vulcanization tube used is the same as a prior art continuous
vulcanization process described above is utilized for the
continuous curing apparatus. The vulcanization tube is 91 meters
(300 feet long), and about two-thirds of the length is filled with
steam at a temperature of 204.degree. C. (400 degrees Fahrenheit)
and a pressure of 1.7 megapascals (250 pounds per square inch).
However, the continuous vulcanization process of the preferred
embodiment of the present invention is operated at a rate of
approximately 23 to 30 meters per minute (75 to 100 feet per
minute) to only partially cure protective jacket 23 as discussed
above, rather than at the prior art rate of speed of between 7.6 to
9.1 meters per minute (25 to 30 feet per minute), which was for
fully curing a prior art elastomeric, protective jacket. This
allows the same manufacturing process line to operate a rate of 30
meters per minute (100 feet per minute), rather than the previous
limit of 7.6 to 9.1 meters per minute (25 to 30 feet per minute),
improving the productivity of the production process. At 23 to 30
meters per minute (75 to 100 feet per minute), and a steam section
of 61 meters (200 feet), a point on the cable will be exposed to
the high temperature for approximately two minutes to two minutes,
forty-five seconds. In the prior art, the exposure to the high
temperature is approximately two to four times more.
Exterior armor 25 is then helically wrapped about protective jacket
23 as depicted in block 47. Then, electric cable 15 is reeled onto
a spool as depicted by block 49. It should be noted that the
preferred method is a continuous process between blocks 41 and 49.
Then, the spooled cable is placed in an oven and heated to a
temperature between 121.degree. C. and 149.degree. C. (250 and 300
degrees Fahrenheit) for between 24 to 36 hours to fully cure
protective jacket 23, as depicted by block 51. This last cure
depicted by block 51 is a batch process, in which a plurality or
spools of cable may be baked simultaneously to cure protective
outer jacket 23.
Referring again to FIG. 2, protective jacket 23 includes an inner
portion 53 and an outer portion 55 which are depicted as separated
by dashed line 57. As illustrated in FIG. 2, protective jacket 23
is partially cured, as depicted in block 45 of FIG. 3, and provides
support to retain plurality of insulated conductors 17 centered
within protective jacket 23 during the manufacturing process steps
of helically wrapping exterior armor 25 about protective jacket 23,
spooling electrical cable 15 onto a reel, and placing electrical
cable 15 within the last oven to completely cure protective jacket
23.
Still referring to FIG. 2, it should be noted that as armor 25 is
wrapped about protective jacket 23, ribs 29 grippingly engage the
interior surface of armor 25 to prevent slipping in a longitudinal
direction along electrical cable 15. Armor 25 will then press
against longitudinally extending ribs 29 causing compressive forces
to arise within protective jacket 23. However, inner portion 53 of
protective jacket 23 is not cured when exterior armor 25 is secured
to protective jacket 23 in the preferred embodiment of the present
invention, allowing these compressive forces to be dispersed
uniformly within inner portion 53. Ribs 29 will flatten to some
extent when wrapped with armor 25, but not completely. After fully
cured, voids 31 will still remain. During thermal expansion within
a well, protective jacket 23 will expand into voids 31.
Referring now to FIG. 4, a fragmentary one-quarter longitudinal
section view depicts a prior art electrical cable 71 which is under
compressive stresses induced by applying an outer armor 73. When
armor 73 is lapped, an outer indentation, or depression 75 is
formed to helically extend into prior art protective jacket 77.
Prior art protective jacket 77 then presses inner indentation 79
into insulation 81, causing a potential weakening of insulation 81
about conductor 83.
Referring now to FIG. 5, a fragmentary one-quarter longitudinal
section view depicts electric cable 15 of the preferred embodiment
of the present invention after application of exterior armor 25. As
depicted in FIG. 5, where armor 25 is lapped by helically wrapping
overlapping end 37 around underlapping end 35, an indentation 85 is
formed into a protective jacket 23, which occurs in outer portion
55. However, inner portion 53 was not fully cured prior to securing
armor 25 about protective jacket 23. In the uncured state, inner
portion 53 was displaced by the compressive forces exerted to cause
depression 85, and uniformly distributed within inner portion 53 so
that there is not an indentation into insulation 21 of insulated
conductor 17.
The present invention provides several advantages over prior art
electrical cables and prior art methods for manufacturing
electrical cables for use in wellbores to conduct electrical power
to downhole submersible pumps. Since the protective jacket of the
present invention is only partially cured during the continuous
process for forming the cable, and then later batch cured to
completely cure the elastomeric protective jacket, the
manufacturing process line for producing the cable may be run
faster. The electric cable is not required to be retained at a high
temperature for curing the elastomeric protective jacket for the
sustained period of time required for a full cure during the
continuous cure process prior to armoring.
Another advantage of the present invention over the prior art is
that compressive stresses induced within the electrical cable by
securing the outer armor about the protective jacket are uniformly
distributed about an inner portion of the protective jacket,
reducing compressive stresses about the insulated conductors.
Additionally, the insulated conductors remain centered with the
protective jacket to provide a sufficient amount of protective
jacket for protecting the insulation about the electrical
conductors from attack by wellbore fluids. Further, thermal
longitudinal expansion voids are spaced uniformly around the
jacket. These voids provide a space for the elastomeric protective
jacket to thermally expand into, reducing compressive forces within
the elastomeric jacket which arise from the protective jacket
expanding within the exterior armor at downhole well
temperatures.
Although the invention has been described with reference to a
specific embodiment, this description is not meant to be construed
in a limiting sense. Various modifications of the disclosed
embodiment as well as alternative embodiments of the invention will
become apparent to persons skilled in the art upon reference to the
description of the invention. It is therefore contemplated that the
appended claims will cover any such modifications or embodiments
that fall within the true scope of the invention.
* * * * *